/*! echo-js v1.7.3 | (c) 2016 @toddmotto | https://github.com/toddmotto/echo */
(function (root, factory) {
    if (typeof define === 'function' && define.amd) {
      define(function() {
        return factory(root);
      });
    } else if (typeof exports === 'object') {
      module.exports = factory;
    } else {
      root.echo = factory(root);
    }
  })(this, function (root) {
  
    'use strict';
  
    var echo = {};
  
    var callback = function () {};
  
    var offset, poll, delay, useDebounce, unload;
  
    var isHidden = function (element) {
      return (element.offsetParent === null);
    };
    
    var inView = function (element, view) {
      if (isHidden(element)) {
        return false;
      }
  
      var box = element.getBoundingClientRect();
      return (box.right >= view.l && box.bottom >= view.t && box.left <= view.r && box.top <= view.b);
    };
  
    var debounceOrThrottle = function () {
      if(!useDebounce && !!poll) {
        return;
      }
      clearTimeout(poll);
      poll = setTimeout(function(){
        echo.render();
        poll = null;
      }, delay);
    };
  
    echo.init = function (opts) {
      opts = opts || {};
      var offsetAll = opts.offset || 0;
      var offsetVertical = opts.offsetVertical || offsetAll;
      var offsetHorizontal = opts.offsetHorizontal || offsetAll;
      var optionToInt = function (opt, fallback) {
        return parseInt(opt || fallback, 10);
      };
      offset = {
        t: optionToInt(opts.offsetTop, offsetVertical),
        b: optionToInt(opts.offsetBottom, offsetVertical),
        l: optionToInt(opts.offsetLeft, offsetHorizontal),
        r: optionToInt(opts.offsetRight, offsetHorizontal)
      };
      delay = optionToInt(opts.throttle, 250);
      useDebounce = opts.debounce !== false;
      unload = !!opts.unload;
      callback = opts.callback || callback;
      echo.render();
      if (document.addEventListener) {
        root.addEventListener('scroll', debounceOrThrottle, false);
        root.addEventListener('load', debounceOrThrottle, false);
      } else {
        root.attachEvent('onscroll', debounceOrThrottle);
        root.attachEvent('onload', debounceOrThrottle);
      }
    };
  
    echo.render = function (context) {
      var nodes = (context || document).querySelectorAll('[data-echo], [data-echo-background]');
      var length = nodes.length;
      var src, elem;
      var view = {
        l: 0 - offset.l,
        t: 0 - offset.t,
        b: (root.innerHeight || document.documentElement.clientHeight) + offset.b,
        r: (root.innerWidth || document.documentElement.clientWidth) + offset.r
      };
      for (var i = 0; i < length; i++) {
        elem = nodes[i];
        if (inView(elem, view)) {
  
          if (unload) {
            elem.setAttribute('data-echo-placeholder', elem.src);
          }
  
          if (elem.getAttribute('data-echo-background') !== null) {
            elem.style.backgroundImage = 'url(' + elem.getAttribute('data-echo-background') + ')';
          }
          else if (elem.src !== (src = elem.getAttribute('data-echo'))) {
            elem.src = src;
          }
  
          if (!unload) {
            elem.removeAttribute('data-echo');
            elem.removeAttribute('data-echo-background');
          }
  
          callback(elem, 'load');
        }
        else if (unload && !!(src = elem.getAttribute('data-echo-placeholder'))) {
  
          if (elem.getAttribute('data-echo-background') !== null) {
            elem.style.backgroundImage = 'url(' + src + ')';
          }
          else {
            elem.src = src;
          }
  
          elem.removeAttribute('data-echo-placeholder');
          callback(elem, 'unload');
        }
      }
      if (!length) {
        echo.detach();
      }
    };
  
    echo.detach = function () {
      if (document.removeEventListener) {
        root.removeEventListener('scroll', debounceOrThrottle);
      } else {
        root.detachEvent('onscroll', debounceOrThrottle);
      }
      clearTimeout(poll);
    };
  
    return echo;
  
  });